1. Field of the Invention
The present invention relates to an active matrix substrate having a driving circuit integrally formed therewith and used for a liquid crystal display device and the like, and a method for inspecting the same.
2. Description of the Related Art
A liquid crystal display device includes an active matrix substrate and a counter substrate opposed to the active matrix substrate with a liquid crystal layer interposed therebetween. A conventional active matrix substrate has a built-in driving circuit as is shown in FIG. 9. In FIG. 9, an active matrix substrate has a base panel 100, a plurality of gate bus lines 101 each acting as a scanning signal line, and a plurality of source bus lines 102 each acting as a data signal line. The gate bus lines 101 and the source bus lines 102 are provided on the base panel 100. The gate bus lines 101 and the source bus lines 102 intersect each other. Each intersection has a pixel capacitance 103 and a pixel transistor 104 as a switching device. In this manner, a plurality of the pixel capacitances 103 and a plurality of the pixel transistors 104 are arranged in a matrix, respectively. Each pixel transistor 104 has a gate electrode connected to the corresponding gate bus line 101, a source electrode connected to the corresponding source bus line 102, and a drain electrode connected to the corresponding pixel capacitance 103. Each pixel capacitance 103 is provided for retaining a video signal supplied thereto through the pixel transistor 104 as a signal charge.
The gate bus lines 101 are each connected to and also driven by a gate driving circuit 105. Among the pixel transistors 104 arranged in a matrix, one row of the pixel transistors 104 is connected to an identical gate bus line 101 and is simultaneously controlled to be on or off by the gate driving circuit 105. The gate driving circuit 105 is constituted by a shift register formed on the base panel 100, and is driven by an external power source and controlled by a start signal, a clock signal or the like externally supplied.
Each source bus line 102 is connected to one of three external signal lines 108 through an analog switch 107 which is controlled to be on or off by a source driving circuit 106.
Among the pixel transistors 104 arranged in a matrix, one column of the pixel transistors 104 is connected to an identical source bus line 102. In this manner, the source bus lines 102 are each connected to one column of the pixel capacitances 103 through the corresponding pixel transistors 104. In detail, each pixel capacitance 103 is connected to the corresponding pixel transistor 104 through one of two electrodes of the pixel capacitance 103. The source bus lines 102 are each connected to an additional capacitance 109 through one of two electrodes of the additional capacitance 109. The other electrode of the additional capacitance 109, which is not connected to the source bus line 102, and the other electrode of the pixel capacitance 103, which is not connected to the pixel transistor 104, are both connected to a wiring 110 and have an identical reference potential with each other.
The source driving circuit 106 is constituted by a shift register formed on the base panel 100, and is driven by an external power source and controlled by a start signal, a clock signal or the like externally supplied. In a case where the active matrix substrate is used for a liquid crystal display device, the three external signal lines 108 receive red, green and blue video signals, respectively, through an external terminal 120. A parasitic capacitance 111 is generated at each external signal line 108.
The active matrix substrate having the above-described configuration is operated in the following manner.
An ON signal is outputted from the gate driving circuit 105 to all the gate bus lines 101 sequentially, thereby turning on each row of the pixel transistors 104 connected to each gate bus line 101 which has received the ON signal. While the ON signal is being outputted to one gate bus line 101, an ON signal is outputted from the source driving circuit 106 to all the analog switches 107 sequentially, thereby turning on the analog switches 107. Then, the source bus line 102 connected to the analog switch 107 which has been turned on gets into connection with the corresponding external signal line 108. Thus, the pixel capacitance 103 connected to each pixel transistor 104 which has been turned on is applied with a signal charge through the source bus line 102.
The signal charge applied to the pixel capacitance 103 is maintained while the gate driving circuit 105 is outputting an ON signal to the gate bus lines 101 which do not correspond to this pixel capacitance 103, since the pixel transistor 104 corresponding to this pixel capacitance 103 is kept off. In this manner, the gate driving circuit 105 outputs an ON signal to all the gate bus lines 101. When a cycle of the above-described procedure is finished, namely, all the gate bus lines 101 have received an ON signal, the same procedure is repeated.
An optical transmittance of a liquid crystal layer depends on an effective voltage applied to each pixel capacitance 103 during one cycle. An improvement in the display quality of the liquid crystal display device is achieved by applying a sufficient level of signal charge to each pixel capacitance 103 and also reducing the amount of leak current to a minimum value so as to maintain the charge applied to the pixel capacitance 103.
The active matrix substrate gets into a drivable state as a part of a liquid crystal display device when combined with a counter substrate with a liquid crystal layer interposed therebetween. Once the active matrix substrate becomes drivable, an optical inspection for any defect can be performed. However, even if the active matrix substrate is judged to have a defect, the defect cannot be corrected unless the active matrix substrate is disassembled. Therefore, the process of assembling the active matrix substrates into a liquid crystal display device is wasted.
A method for electrically inspecting an active matrix substrate concerning operation of each pixel before the assembly into a liquid crystal display device is effective for improving the yield and reducing the product cost, since such a method, for example, eliminates unnecessary disassembly and reassembly process which would be necessary after the inspection with a conventional inspection method and allows the defect to be corrected relatively easily.
Such an electrical method is performed by use of an inspection apparatus shown in FIG. 10.
FIG. 10 is an equivalent circuit diagram for applying a charge corresponding to an inspection signal to one pixel capacitance 103 and detecting the level of the signal from the pixel capacitance 103. The inspection apparatus is connected to the external terminal 120 of the active matrix substrate. The inspection apparatus includes a switch 116 through which an inspection signal is inputted to the active matrix substrate, a switch 112 connected to the external terminal 120, a buffer 113 connected to the external terminal 120 through the switch 112, an A/D converter 114 for receiving a signal from the buffer 113, and a computer 115 for receiving a signal from the A/D converter 114.
The active matrix substrate is inspected in the following manner.
The switch 116 is turned on to supply the external terminal 120 with a certain potential as an inspection signal, and in this state, the gate driving circuit 105 and the source driving circuit 106 (FIG. 9) are driven. When one row of the pixel transistors 104 and the corresponding analog switches 107 are both turned on, a signal charge is sequentially applied to each of the pixel capacitances 103 corresponding to the pixel transistors 104 and the analog switches 107 which have been turned on. After the signal charge is kept for a certain period of time, the switch 112 is turned on to drive again the gate driving circuit 105 and the source driving circuit 106. When the same row of the pixel transistors 104 and the corresponding analog switches 107 are both turned on, the level of the signal corresponding to the signal charges which have been retained in the corresponding pixel capacitances 103 are sequentially detected through the external terminals 120, and amplified by the buffer 113 to be inputted to the computer 115 through the A/D converter 114. If an abnormality is generated with at least either one of the applying operation of the signal charge to the pixel capacitance 103 or the retaining operation of the signal charge in the pixel capacitance 103, the level of the signal corresponding to the signal charge is not detected. Thus, the active matrix substrate is judged to have a defect. Further, since the level of the signals are sequentially detected, the position where the defect exists can also be detected based on the timing and the distribution shape of the pulses of the detected signal. Such an inspecting method is proposed in, for example, Japanese Patent Publication No. 1-36118 and Japanese Laid-Open Patent Publication No. 64-9375.
In a conventional active matrix substrate, the pixel capacitance 103 is small compared with the additional capacitance 109 at the source bus line 102 and the parasitic capacitance 111 at the external signal line 108. Accordingly, the level of the signal charge to be detected is low, thus making it difficult to measure the level of the signal charge. For example, where the pixel capacitance 103 is 0.2 pF, the additional capacitance 109 is 5 pF, and the parasitic capacitance 111 is 15 pF, when a voltage of 5 V is inputted from the external terminal 120 as an inspection signal to apply the pixel capacitance 103 with a signal charge, a change in the potential detected from the external terminal 120 is 50 mV per pixel, which is extremely small.
Since the signal charge applied to the pixel capacitance 103 has a small absolute value and thus the signal detected from the external terminal 120 is easily influenced by noise, it is difficult to ensure a sufficient S/N ratio.
The switches 112 and 116 which are externally provided have problems in that the input capacitance of the switches 112 and 116 is large compared with the pixel capacitance 103 and thus the level of the signal detected from the external terminal 120 is further lowered, and that a fluctuation in the potential of the external signal line 108 accompanied by the ON/OFF operation of the switches 112 and 116 is large.
Another problem concerns rubbing treatment. An active matrix substrate which has been judged satisfactory or which has been subjected to correction is sent to a process of forming an image forming section of the liquid crystal display device. In this process, rubbing treatment is performed to align liquid crystal molecules as a display medium. For such rubbing treatment, an alignment film of polyimide or the like formed on the active matrix substrate is rubbed by a cloth, which generates a large amount of static electricity, Static electricity may be generated in other processes for producing a liquid crystal display device such as a coating process and a seal printing process. If such a process for producing a liquid crystal display device is performed after the above-described electrical inspection, the pixel transistors 104 are possibly broken by the large amount of static electricity, resulting in an increase in defects.